Karin,
I know I'm giving you an answer that you're probably not looking for, but just keep jumping rope the way you are and keep trying. It will come, trust me. I started out the same way; one double under, one single and then a double under again, etc. etc. After awhile it just seemed to not feel right. Then I watched a video of someone on this website (sorry, can't remember where or of whom) doing correct, consecutive DU's and it just clicked. I actually didn't even know for the longest time I was doing them the wrong way. Now, I don't even think I could do them the wrong way if I tried, it's actually hard for me to do regular singles now. Keep playing with the movement and make sure your jumping on some sort of floor mat or padding, not concrete. I'd say start slow, but DU's are not slow in general. They're supposed to be fast. Just start off trying to get two DU's in a row without a single in between and slowly build you reps of DU's from there. If you all ready have DU's down to some degree (which it sounds like you do) then a good solid week of consistent practice will put you well on your way to performing them consecutively with no problem. Once you become good at them, it's hard to find a rival to them as a conditioning tool. They are phenomenal at building speed and stamina. If you have any other questions, don't hesitate to ask. Good luck!